A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.
Lithography is widely recognized as one of the key steps in the manufacture of ICs and other devices and/or structures. However, as the dimensions of features made using lithography become smaller, lithography is becoming a more critical factor for enabling miniature IC or other devices and/or structures to be manufactured.
A theoretical estimate of the limits of pattern printing can be given by the Rayleigh criterion for resolution as shown in equation (1):
                    CD        =                              k            1                    *                      λ                          NA              PS                                                          (        1        )            where λ is the wavelength of the radiation used, NAPS is the numerical aperture of the projection system used to print the pattern, k1 is a process dependent adjustment factor, also called the Rayleigh constant, and CD is the feature size (or critical dimension) of the printed feature. It follows from equation (1) that reduction of the minimum printable size of features can be obtained in three ways: by shortening the exposure wavelength λ, by increasing the numerical aperture NAPS or by decreasing the value of k1.
In order to shorten the exposure wavelength and, thus, reduce the minimum printable size, it has been proposed to use an extreme ultraviolet (EUV) radiation source. EUV radiation sources are configured to output a radiation wavelength of about 13 nm. Thus, EUV radiation sources may constitute a significant step toward achieving small features printing. Such radiation is termed extreme ultraviolet or soft x-ray, and possible sources include, for example, laser-produced plasma sources, discharge plasma sources, or synchrotron radiation from electron storage rings.
A property of EUV radiation is that all known materials are highly opaque to EUV radiation. As a consequence, reflective elements instead of lenses are used in any optical systems of the lithographic projection apparatus. For this reason, the patterning device is typically also reflective instead of transmissive.
A transmissive optical element that cannot easily be replaced by a reflective one when using EUV radiation, is a pellicle. Pellicles are known in the art, for instance from United States Patent Application Publication No. US2005/0280789. Contamination deposits on the pellicle, while radiation transmits through the pellicle. Since the pellicle is located at a certain predetermined distance from the patterning device, the contamination will be out-of-focus and thus will not be projected onto the target portion of the substrate. Because the transmissivity of the pellicle to the radiation that is to be patterned contributes to the proper functioning of the pellicle, a pellicle cannot be used when EUV is being used. Therefore, a pellicle does not provide acceptable solution to contamination deposition on the patterning device.